A portable two-way radio device is a hand-held device that the user carries on the user's person, such as in a holster or other carrier, and the user can operate the portable two-way radio by, for example, manipulating buttons and knobs on the radio, or by using a remote accessory also worn by the user that is connected to the portable two-way radio device. Portable two-way radios are used in a wide variety of applications where instant or near-instant communication is desirable. Two-way communication systems, including portable two-way radio devices, are used by public safety, emergency, rescue, fire, and industrial organizations, among others. Generally, two-way communication is a half-duplex type of communication where the radio device is either receiving, transmitting, or in standby/monitoring mode when on. When a user “keys up” a two-way radio, the radio will typically check to make sure the channel is clear and then begin transmitting after giving an audible indication to the user indicating the radio is transmitting and the user can commence speaking. The radio continues to transmit as long as the user has the radio keyed. To key the radio the user can press a “push to talk” button on the radio, or a remote radio microphone accessory, if used.
In some situations, the user of a portable two-way radio device may carry the radio, knowingly or unknowingly, into places where there are volatile gasses in the surrounding air. Examples of such places including mining operations, industrial and chemical settings, accident scenes, and so on. Given that portable two-way radio devices are battery powered, and contain various components that store energy in addition to the battery (e.g. capacitors, inductors), the potential exists for a discharge event or localized heat generation that can cause ignition of surrounding volatile gasses. To address such conditions manufacturers have taken measures to design portable two-way radio devices that are classified as being intrinsically safe for use by personnel who could operate in such conditions. To be rated as intrinsically safe, among other limitations, a battery powered electronic device must be designed to limit energy storage in the circuit components in case a fault occurs that releases the energy stored in those components, and precautions must be taken to limit energy available at the battery terminals to prevent sufficiently energetic sparks in case an inadvertent shorting between the terminals occurs. Furthermore, power provided to the device or components in the device from the battery may need to be limited in order to prevent thermal heating of small components to avoid creating a heat source sufficient to cause ignition of volatile gasses, dust, or other such compounds in the surrounding atmosphere.
The typical storage components in a given electronic device are capacitors and inductors. Capacitors store energy in the form of electrical charge and inductors store energy in the form of a magnetic field resulting from current through the inductor. Most of these passive components are small in typical electronic devices, including two-way radios, and will not store sufficient energy to be of concern, even when summed together under the assumption of an extreme fault condition. However, power handling components can have capacitance or inductance values large enough to be of concern, and may need to be limited.
In portable two-way radio devices, audio volume is particularly important since the user must be able to hear communications under potentially noisy conditions. The need for audio volume sufficient for the user to hear received audio over ambient sounds necessitates the use of speakers to play received audio signals. Speakers suitable for portable two-way radio applications typically use an inductive speaker coil that is driven by the audio amplifier. In an intrinsically safe application, however, the inductance of the speaker coil has to be limited to prevent exceeding energy storage specifications as well as avoiding excessive temperatures. Limiting the speaker coil inductance, though, limits the audio volume that can be achieved.
Accordingly, there is a need for an apparatus for achieving a desired audio volume without exceeding specified energy storage and temperature limits in an audio circuit of a device designed for intrinsically safe applications.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.